Not All Cats With Big Brains Are Social Creatures

Wild cats have similar-sized brains, despite the fact that some species are social whereas others are solitary. This has implications for one of nature's most fascinating questions:

Why are some animals smarter than others?

The leading theory is that intelligence evolved in certain species so they could deal with the complex social relationships associated with group living. Living in groups introduces competition within a society, so individuals must use tactics like cooperation or deception to survive. Understanding all those interactions requires mental processing power, which would drive natural selection for a big brain -- especially a large frontal cortex, the region responsible for advanced cognitive skills. This is the "social brain" hypothesis.

Group size is related to cortex size in mammals, and many intelligent species -- such as elephants and dolphins -- live in large societies. But although there's plenty of support for the social brain hypothesis, new evidence shows that evolutionary pressure for a social brain isn't the only force behind intelligence.

Average brain size doesn't vary among cat species, according to a study led by neuroscientist Sharleen Sakai of Michigan State University. Her team did computed tomography (CT) scans on 75 skulls obtained from museums, representing 13 wild feline species, then used software to calculate brain volumes. Two species -- lions and cheetahs -- are social while the remaining 11, including leopards, are solitary creatures.

When comparing species, the social brain hypothesis doesn't seem to hold true. Leopards are solitary creatures but their frontal cortex is relatively large, for example, whereas social cheetahs have the smallest cortex. (Cheetahs may be special though. "Cheetah brain anatomy is distinctive and differs from other wild cats," Sakai said. "The size and shape of its brain may be a consequence of its unusual skull shape, an adaptation for high-speed pursuits.")

Cats illustrate that not all creatures became clever because they needed social brains. And even if the hypothesis is generally true, there are a few exceptions. In primates, for instance, monkeys live in larger groups than apes, but aren't as smart. Meanwhile, the social brain may apply within a cat species: female lions have a larger frontal cortex and are more social than males, which may live alone when not part of a pride.

Intelligence has clearly evolved for various reasons in mammals. As Sakai said, "Our findings suggest the factors that drive brain evolution in wild cats are likely to differ from selection pressures identified in primate brain evolution."